Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
In thermography two approaches are known:
1. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
2. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
Thermal dye transfer printing is a recording method wherein a dye-donor element is used that is provided with a dye Layer wherefrom dyed portions or incorporated dyes are transferred onto a contacting receiver element by the application of heat in a pattern normally controlled by electronic information signals.
A survey of "direct thermal" imaging methods is given e.g. in the book "Imaging Systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), Chapter VII under the heading "7.1 Thermography". Thermography is concerned with materials which are substantially not photosensitive, but are sensitive to heat or thermosensitive. Imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material.
Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
A wide variety of chemical systems has been suggested some examples of which have been given on page 138 of the above mentioned book of Kurt I. Jacobson et al., describing the production of a silver metal image by means of a thermally induced oxidation-reduction reaction of a silver soap with a reducing agent.
As described in "Handbook of Imaging Materials", edited by Arthur S. Diamond--Diamond Research Corporation--Ventura, California, printed by Marcel Dekker, Inc. 270 Madison Avenue, New York, N.Y. 10016 (1991), p. 498-499 in thermal printing image signals are converted into electric pulses and then through a driver circuit selectively transferred to a thermal printhead. The thermal printhead consists of microscopic heat resistor elements, which convert the electrical energy into heat via Joule effect. The electric pulses thus converted into thermal signals manifest themselves as heat transferred to the surface of the thermal paper wherein the chemical reaction resulting in colour development takes place.
According to U.S. Pat. No. 3,080,254 a typical heat-sensitive copy paper includes in the heat-sensitive layer a thermoplastic binder, e.g ethyl cellulose, a water-insoluble silver salt, e.g. silver stearate and an appropriate organic reducing agent, of which 4-methoxy-1-hydroxy-dihydronaphthalene is a representative. Localized heating of the sheet in the thermographic reproduction process, or for test purposes by momentary contact with a metal test bar heated to a suitable conversion temperature in the range of about 90.degree.-150.degree. C., causes a visible change to occur in the heat-sensitive layer. The initially white or lightly coloured layer is darkened to a brownish appearance at the heated area. In order to obtain a more neutral colour tone a heterocyclic organic toning agent such as phthalazinone is added to the composition of the heat-sensitive layer. Thermo-sensitive copying paper is used in "front-printing" or "back-printing" using infra-red radiation absorbed and transformed into heat in contacting infra-red light absorbing image areas of an original as illustrated in FIGS. 1 and 2 of U.S. Pat. No. 3,074,809.
U.S. Pat. No. 3,241,997 concerns a heat-sensitive copying material having two separate layers located one on top of the other and carried by a supporting material, the layers having different melting points and being soluble in different solvents, the solvent of one layer being incapable of dissolving the other layer, the layers containing at least two different chemical reagents capable of reacting with each other when at least one of them is in molten form to produce colour, the colour-producing reagents being located in two different layers, whereby each layer contains at least one colour-producing reagent and whereby no one layer contains all of the colour-producing reagents, at least one of the layers and at least one of the colour-producing reagents melting at most at 150.degree. C.
U.S. Pat No. 3,795,532 concerns sheet material containing metal soap reactants and adapted for producing a copy of an original in a heat-activated copying process when associated with a coreactant source to form a couple. The coreactant may be provided as a separate coating directly overlying and bonded to the soap layer.
A heat-sensitive recording material containing silver behenate and 4-methoxy-1-naphthol as reducing agent in adjacent water-insoluble polymeric binder layers is also described in Example 1 of U.S. Pat. No. 3,094,417.
Further, the separate application in a thermosensitive recording material of an organic silver salt and hydroxylamine type reductor in thermal working relationship in adjacent layers containing a thermoplastic water-insoluble binder such as ethyl cellulose and after-chlorinated polyvinyl chloride is described already in U.S. Pat. No. 4,082,901.
In a special embodiment of direct thermal imaging a heat-sensitive recording material is used in the form of an electrically resistive ribbon having a multilayered structure in which a carbon-loaded polycarbonate is coated with a thin aluminium film (ref. Progress in Basic Principles of Imaging Systems--Proceedings of the International Congress of Photographic Science Koln (Cologne), 1986 ed. by Friedrich Granzer and Erik Moisar--Friedr. Vieweg & Sohn--Braunschweig/Wiesbaden, FIG. 6. p. 022). Current is injected into the resistive ribbon by electrically addressing a print head electrode contacting the carbon-loaded substrate, thus resulting in highly localized heating of the ribbon beneath the energized electrode.
The fact that in using a resistive ribbon recording material heat is generated directly in the resistive ribbon and only the travelling ribbon gets hot (not the print heads) an inherent advantage in printing speed is obtained. In applying the thermal printing head technology the various elements of the thermal printing head get hot and must cool down before the head can print without cross-talk in a next position.
In another embodiment of direct thermal imaging the recording material is image-wise or pattern-wise heated by means of a modulated laser beam. For example, image-wise modulated infra-red laser light is absorbed in the recording layer in infra-red light absorbing substances converting infra-red radiation into the necessary heat for the imaging reaction.
The image-wise applied laser light has not necessarily to be infrared light since the power of a laser in the visible light range and even in the ultraviolet region can be thus high that sufficient heat is generated on absorption of the laser light in the recording material. There is no limitation on the kind of laser used which may be a gas laser, gas ion laser, e.g. argon ion laser, solid state laser, e.g. Nd:YAG laser, dye laser or semi-conductor laser.
The use of an infrared light emitting laser and a dye-donor element containing an infrared light absorbing material is described e.g. in U.S. Pat. No. 4,912,083. Suitable infra-red light absorbing dyes for laser-induced thermal dye transfer are described e.g. in U.S. Pat. No. 4,948,777, which U.S. Pat. No. documents for said dyes and lasers applied in direct thermal imaging have to be read in conjunction herewith.
The image signals for modulating the laser beam or current in the micro-resistors of a thermal printhead are obtained directly e.g. from opto-electronic scanning devices or from an intermediary storage means, e.g. magnetic disc or tape or optical disc storage medium, optionally linked to a digital image work station wherein the image information can be processed to satisfy particular needs.
When used in thermographic recording operating with thermal printheads said recording materials will not be suited for reproducing images with fairly large number of grey levels as is required for continuous tone reproduction.
According to EP-A 622 217 relating to a method for making an image using a direct thermal imaging element, improvements in continuous tone reproduction are obtained by heating the thermal recording element by means of a thermal head having a plurality of heating elements, characterized in that the activation of the heating elements is executed line by line with a duty cycle .DELTA. representing the ratio of activation time to total line time in such a way that the following equation is satisfied: EQU P.ltoreq.P.sub.max =3.3W/mm.sup.2 +(9.5 W/mm.sup.2 .times..DELTA.)
wherein P.sub.max is the maximal value over all the heating elements of the time averaged power density P (expressed in W/mm.sup.2) dissipated by a heating element during a line time.
Although by controlling the heating of the heating elements of a thermal head in the way as described in said EP-A already a substantial improvement in continuous tone reproduction is obtained, from the side of the composition of the thermal recording element further improvements to lower the image gradation are still desirable.